Advances in manufacturing technology related to the production of metal parts have led to the development of machine tools known as machining centers. Machining centers are sophisticated milling machines that are computer numerically controlled (CNC) and are capable of performing complex milling, boring and drilling operations with great accuracy and precision. The computer control allows these machines to be highly automated and they are capable of performing several successive operations on a part completely automatically.
These machine tools include a spindle which rotates about a central axis and which can be advanced and withdrawn along this axis. The spindle carries a tool engaging fixture capable of supporting any one of a variety of cutting tools. The workpiece is secured to a movable table that can move the part relative to the tool in directions perpendicular to the central axis. The part is brought into contact with the rotating tool by moving the table and/or the spindle and is thereafter machined to the desired shape and size.
A variety of tools is available to perform the various machining operations. Such tools include reamers, taps, end mills, slab mills and drill bits. To allow successive operations using these various tools to proceed automatically, however, it is necessary to transfer individual tools between the tool engaging fixture of the spindle and a tool storage magazine containing the various tools. At the completion of one operation, the tool is disengaged from the fixture and is transferred from the fixture to the magazine. A second tool is then transferred from the magazine to the fixture and a second machining operation can proceed.
Conventional machine tools often use an automatic tool changer to transfer individual tools between the spindle and the magazine. The tool storage magazine is rotatably mounted at a position withdrawn from the spindle axis and includes a plurality of tool holders arranged thereon. The tool storage magazine is rotated until a desired tool holder occupies a transfer position and the tool in that tool holder is then transferred to the spindle. The transfer position is fixed relative to the tool changer axis so that any one of the tool holders can be moved into the transfer position by rotation of the magazine.
One type of conventional tool changer typically includes one or more rotatably mounted transfer arms for transferring tools between the spindle and the tool holder at the transfer position of the magazine. In operation, a tool is removed from the spindle by the transfer arm and is transferred to an empty tool holder at the transfer position of the magazine. The magazine is then rotated until the tool holder carrying the desired tool occupies the transfer position and the transfer arm then transfers this tool to the spindle.
Other conventional tool changers obviate the need for a separate transfer arm by mounting the tool storage magazine on a trackway for moving the magazine between a withdrawn position and an advanced position. When the magazine is moved to its advanced position, the axis of the transfer position of the tool storage magazine coincides with the central axis of the spindle. The tool engaging fixture of the spindle can be advanced and withdrawn along the central axis, as described above, to selectively engage and disengage a tool in the tool holder of the magazine.
The operation of transferring a tool begins with the magazine in the withdrawn position and an empty tool holder occupying the transfer position. The tool engaging fixture is advanced along the spindle axis from the withdrawn position to an intermediate position. The magazine is advanced along the trackway until the empty tool holder engages the tool and the fixture then disengages the tool, thereby depositing the tool in the tool holder. The fixture is withdrawn and the magazine rotates until the desired tool occupies the transfer position. The fixture is then advanced to the intermediate position, engages the desired tool, and the tool storage magazine is withdrawn. The fixture and spindle are then further advanced to the engage the workpiece.
These conventional automatic tool changers greatly increase the productivity of machine tools, however, each of the conventional tool changers requires the use of at least two separate drive sources of either rotational or linear force. For example, the first above-described conventional tool changer may use an electric motor to rotate the tool storage magazine and a separate electric motor to rotate the transfer arm. Similarly, the second above-described machine may use an electric motor to rotate the magazine and a hydraulic cylinder to translate the magazine between the withdrawn and advanced positions. Thus, both types of conventional tool changers require the use of more than one drive source.
The use of separate drive sources in machine tools to drive the automatic tool changers is costly and complex. In addition to the physical space requirements and cost of the separate drive sources, the control system necessary to coordinate the functions of the two or more drive sources is by necessity quite complex.
It is therefore an object of the present invention to provide an improved automatic tool changer for machine tools.
It is still another object of the present invention to provide an improved automatic tool changer for machine tools wherein only a single drive source is used.